[Campbell Biology P.383] E. coli cells grown on 15N medium are transferred to 14N medium and... | Practice Question

E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations two rounds of DNA replication . DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment?

  • A: one high-density and one low-density band
  • B: one intermediate-density band
  • C: one high-density and one intermediate-density band
  • D: one low-density and one intermediate-density band

Explanation

Based on the semi-conservative model of DNA replication, after one generation in 14N medium, all DNA molecules will have one 15N strand and one 14N strand, resulting in a single intermediate-density band. After a second generation two rounds total , each of these intermediate molecules will replicate. This will produce half of the DNA as intermediate density one 15N strand and one 14N strand and the other half as low density two 14N strands . Therefore, two bands would be observed: one low-density band and one intermediate-density band.